1. Field of the Invention
This invention relates to turbofan gas turbine engines, and more specifically to supercharging of working medium gases flowed through the core section of such an engine.
2. Description of the Prior Art
Turbofan, gas turbine engines are the type of powerplants most widely used in large aircraft today. In turbofan engines, as distinguished from turbojet engines, a portion of the working medium is pumped axially through one or more compression stages and is exhausted to the atmosphere without passing through the core section of the engine. Such compression stages are called fan stages.
In the least complex of turbofan engines, the stages of the fan and the stages of the core are driven by separately rotating shafts. The shaft driving the fan stages rotates at a speed slower than the shaft driving the core stages. The ratio of the air flowing through the fan stages alone to the air flowing through the core stages is referred to as the bypass ratio. The bypass ratio may be a different value for each individual engine model according to the performance requirements of that powerplant. In all turbofan engines, however, the fan stages make a substantial contribution to the total engine thrust at take-off.
For large thrust contributions a bypass ratio of five (5) or greater is typical. At these bypass ratios the diameter of the fan need be very large to pass the required amount of working medium. In such a configuration, the root region of each fan blade is of necessity closely spaced to the root region of the adjacent blade in order that the tip regions of the blades are not excessively spaced. The root regions of the blades have a relatively short chord length and a minimal twist. Also, the blade speed relative to the incoming medium is significantly less in the root region. Resultantly, the root portions of the blades have a limited capacity to raise the pressure of the medium pumped thereby. In a typical turbofan engine, the pressure ratio attainable across the fan blades in the root region is only a ratio of about one and five tenths (1.5) in contrast to the pressure ratio attainable across the tip regions of the blades which is a ratio of approximately one and seven tenths (1.7).
To compensate for the reduced capacity of the fan blades in the root regions, modern engines utilize one of two configurations for raising the pressure of the medium approaching the core stages of the engine. A first approach is embodied in representative U.S. Pat. No. 3,283,995 to Fligg, Jr. entitled "Splitter Vane Construction for Turbofan Engine". In this turbofan configuration, low compression or supercharging stages are mechanically coupled to the fan stage immediately downstream of the fan stage. A core case circumscribes the low compression stages and extends into close proximity to fan blades. The inward portion of the working medium gases discharged by the fan stage is captured by the core case and is directed into the low compression stages.
In a second approach illustrated by U.S. Pat. Nos. 3,494,129 to Krebs et al entitled "Fluid Compressors and Turbofan Engines Employing Same"; 3,528,246 to Fischer entitled "Fan Arrangement for High Bypass Ratio Turbofan Engine"; and 3,536,414 to Smith, Jr. entitled "Vanes for Turning Fluid Flow in an Annular Duct", short fan blades extend outwardly to a flow splitter disposed across the fan stream. The flow splitter extends into close proximity to the fan blades. Flow treated by the inward portions of the fan blades is confined by the splitter and subsequently treated in total by the short blades.
Although the two turbofan configurations discussed above have significant structural dissimilarities, both configurations aerodynamically couple the low compression, or supercharging stages to the fan stages. In the former, the core case extends into close proximity with the fan blades causing the working medium discharged by the inward portion of the fan stage to be directed in total into the low compression stages. In the latter, the flow splitter extends into close proximity with the fan blades causing the working medium discharged by the fan inwardly of the splitter to be directed in total into the short blades forming the supercharging, compression stages. Such series treatment of common working medium gases is referred to as "aerodynamic coupling", and in both configurations the supercharging stage is aerodynamically coupled to the fan stage.
Although the configurations represented by this prior art are high technology structures providing adequate service in th aviation field today, scientists and engineers continue to search for yet improved configurations which will enhance performance and product reliability.